Wireless communication is characterized as transmitting signals between two locations without the use of a connection cable. In some instances, the two locations may be rather distant from each other. Wireless communication may provide a convenient option for communication between such distant locations.
Equipment for the wireless communication system has typically been rather expensive. However, recently, equipment costs have declined due to recent developments in semiconductor fabrication, wireless communication technology, and digital signal processing. Thus, applications related to wireless communications are increasing. For example, wireless local area networks (LAN) has recently emerged as a viable alternative to traditional wired-LAN.
In typical wireless communication systems, data or information is converted into a radiofrequency (RF) signal prior to transmission to a remote receiver. The remote receiver receives the RF signal and processes the received RF signal to recover the originally transmitted data or information. Many approaches have been proposed to assure proper transmission and recovery of the data or information. Systems and methods relating to space-time block code (STBC) are included among these approaches.
STBC has been proposed to provide transmit diversity gain to a receiver with a very simple maximum-likelihood decoding algorithm. FIG. 5 is a schematic illustration showing the operating principle of STBC. As shown in FIG. 5, an STBC coder 500 is associated with two transmission antennas 502, 504 to achieve transmitter diversity. Once signals have been transmitted from the two transmission antennas 502, 504, a receiving antenna 506 receives the substantially simultaneously transmitted signals. In FIG. 5, the signals, channel impulse responses, and noise are shown in the time domain, which are conventionally denoted by lower-case letters. An input complex-symbol stream of the STBC encoder 500 is grouped into pairs of signals, which are denoted as x1 and X2. Each symbol has a symbol period of T, with x1 being the first symbol and x2 being the second symbol that follows x1. After receiving x1 and x2, the STBC encoder 500 performs a two-step calculation, with each calculation outputting one symbol at each of the two output ports of the STBC encoder 500. At the first step of the calculation, the STBC encoder 500 simply relays x1 and X2 to its first parallel output port 512 and second parallel output port 514, Then x1 and x2 are directed to individual RF transmitters (not shown) to be converted to RF signals for transmission by antennas 502, 504. At the second step of the calculation, −x2* (which is the negative of the complex conjugate of x2) and x1* (which is the complex conjugate of x1) are generated by the STBC encoder 500 at the first parallel output port 512 and second parallel output port 514, respectively. It is presumed, in the STBC model, that the gain h1 of the channel between the first antenna 502 and the receiving antenna 506 is independent of the gain h2 of the channel between the second antenna 504 and the receiving antenna 506. It is further presumed that both channels are static across two consecutive transmit symbols. In other words, it is presumed that the channel gain for the second symbol period is the same as the channel gain for the first symbol period. Thus, if y1 and y2 represent the received symbols at the first and second symbol periods, respectively, then y1 and y2 may be expressed as:y1=h1·x1+h2·x2+z1   [Eq. 1],and:y2h1·(−x2*)+h2·x1*+z2   [Eq. 2],where z1 and z2 are independent additive noise terms. If {tilde over (h)}1 and {tilde over (h)}2 are the estimated channel gains of h1 and h2, respectively, then approximations (within a multiplicative factor) of the two transmit signals {circumflex over (X)}1 and {circumflex over (X)}2 can be approximately recovered in terms of {tilde over (h)}1 and {tilde over (h)}2 as:{circumflex over (x)}1={tilde over (h)}1*·y1+{tilde over (h)}2·y2*   [Eq. 3],and:{circumflex over (x)}2={tilde over (h)}2*·y1−{tilde over (h)}1·y2*   [Eq. 4].
Though the above STBC is described in space-time domain, it is clear to one of ordinary skill in the art that the STBC can also be applied to space-frequency domain. Since the principles underlying STBC are well known, further discussion of STBC principles is omitted.
STBC is optimally designed for systems that employ channels which are flat-fading and time-invariant over two consecutive symbol durations. Since time-invariant flat-fading channels have a negligible variation in channel gain, systems employing flat-fading channels may be modeled as constant-gain systems. Unfortunately, if STBC is employed in a wide-band system, the constant-gain system model is no longer accurate because the channel gain within the bandwidth may vary considerably. Hence, for wide-band systems, inter-symbol interference (ISI) mitigation techniques, such as frequency-selective channels, are typically used.
In systems employing frequency-selective channels, each channel may be viewed as having a constant gain due to the division of the wide frequency band into multiple narrower frequency channels. Thus, while inter-channel distortions may vary across the wide frequency band, intra-channel distortions within the narrower frequency channels may be presumed to be constant. Thus, the ISI may be independently determined for each channel.
In mitigating ISI problems, orthogonal frequency division multiplexing (OFDM) techniques have shown promise. OFDM techniques have only recently gained popularity, due in part to advances in signal processing and microelectronics. OFDM splits data streams into N parallel data streams of reduced data rate, and transmits each of the N parallel data streams on a separate sub-carrier. The sub-carriers are made orthogonal to each other by appropriately choosing the frequency spacing between the sub-carriers. Therefore, since the orthogonality of the OFDM sub-carriers typically ensures that the receiver can separate the OFDM sub-carriers, spectral overlapping among sub-carriers may be permitted.
OFDM operation block diagrams are shown in FIG. 6. Following convention, the signals, the channel impulse responses, and the noise in the frequency domain are denoted by capital letters while the signals, the channel impulse responses, and the noise in time domain are denoted by a lower-case letters. As shown in FIG. 6, the serial-to-parallel (S/P) converter 600 collects frequency-domain serial input signals X(k), where k={0, . . . , N−1}, and N is the number of OFDM sub-carriers. The collected frequency-domain serial input signals X(k) are converted into frequency-domain parallel signals. The frequency-domain parallel signals are then directed to an inverse discrete Fourier transform (IDFT) circuit 602. The IDFT circuit 602 transforms the frequency-domain parallel signals into time-domain parallel signals. The time-domain parallel signals from the IDFT circuit 602 are then converted into time-domain serial signals, x(n), where n={0, . . . , N−1}, by a parallel-to-serial (P/S) converter 604. Then, a cyclic-prefixed time-domain signal xCP(n) is generated in the CP unit 606 by adding a cyclic prefix (CP) with a guard period of G. The length of the CP is a value longer than the channel delay spreading. Thus:
                                          x            ⁡                          (              n              )                                =                                    1                              N                                      ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  ⁡                                      (                    k                    )                                                  ⁢                                  ⅇ                                      j                    ⁢                                                                  2                        ⁢                                                                                                  ⁢                        π                        ⁢                                                                                                  ⁢                        k                        ⁢                                                                                                  ⁢                        n                                            N                                                                                                          ,                  n          =          0                ,                                  ⁢        …        ⁢                                  ,                  N          -          1                ,                            [                  Eq          .                                          ⁢          5                ]            andxCP(n)=x((n+N−G)mod N)   [Eq. 6],with CP being a part of the original packet. Since N+G signals are actually transmitted, the signal block becomes xCP(n), where n={0, . . . , N+G−1}. RF unit 608 converts the signal block xCP into the RF signal, which is then transmitted by antenna 610. Antenna 612 at the receiver side subsequently receives the signal. After being converted by the RF unit 614, the received signal yCP(n) becomes:yCP(n)=xCP(n){circle around (×)}h(n)+z(n), n=0, . . . , N+G−1   [Eq. 7]where {circle around (×)} denotes convolution operator, h(n) is the time domain channel impulse response of the channel from the antenna 610 to the antenna 612, and z(n) represents the noise. After removing cyclic prefix at the CP unit 616, the resulting signal y(n) becomes:y(n)=yCP((n+G)mod N), n=0, . . . , N−1   [Eq. 8]Thereafter, y(n) is directed through a S/P converter 618. The resulting signals are fed into a discrete Fourier transform (DFT) circuit 220, which transforms time-domain signals into frequency domain signals. The resulting output Y(k) of the DFT circuit 620, where k={0, . . . , N−1} is:Y(k)=X(k)·H(k)+Z(k)   [Eq. 9]where H(k) and Z(k) are the N-dimensional DFT of h(n) and z(n), respectively. The parallel output of the DFT circuit 620 is then connected to a P/S converter 622, which generates serial signals.
Since the principles underlying OFDM technology is well known, further discussion of OFDM principles is omitted.
While STBC-OFDM technology has been proposed for wireless communication systems, the complexity and cost impediments of implementing STBC-OFDM to wireless communication systems is still relatively high.
In typical OFDM systems with two-branch transmitter diversity using STBC, a data block X is encoded by the STBC encoder into two parallel signal blocks XA and XB, each having N data elements. XA and XB are then inverse Fourier transformed to produce time-domain signals XA and XB, respectively. Cyclic prefixes are then added to each time-domain domain signal to produce cyclic-prefixed time-domain signals XA,cp and XB,cp, respectively. Each of the cyclic-prefixed time-domain signals forms an OFDM block. The OFDM blocks are converted into RF signals and transmitted from their respective transmitter antennas simultaneously. Since two transmitter antennas are employed for the transmission of the signals, each of the signals is altered by channel characteristics hA and hB of their respective channels. As mentioned above, it is assumed that the channel characteristics are time-invariant during the period of an OFDM block. Thus, when a receiver antenna receives the aggregate signal from both of the transmitters, the received aggregate signal y(n) may be seen as:ycp(n)=(hA(n){circle around (×)}xA,cp(n))+hB(n){circle around (×)}xA,cp(n))+z(n) [Eq. 10],where n is the discrete time index, {circle around (×)} represents a convolution function, and z(n) represents noise in the system. The received signal y(n) is produced upon removing the cyclic prefix of ycpp(n) according Eq. 6. The received signal may be represented in the frequency domain as:Y(k)=(HA(k)·XA(k))+(HB(k)·XB(k))°Z(k), k=0, . . . , N−1   [Eq. 11],where Y(k), H(k) and Z(k) are the N-dimensional DFT of y(n), h(n) and z(n), respectively. Eq. 11 shows the received signal Y(k) as a superposition of the two transmitter signals and the noise.
Thus, for two-branch STBC OFDM systems, a first pair of signals xA=x1 and xB=x2 are generated, and, after appropriate processing, are transmitted from the first and second antennas, respectively. Upon receiving these signals, a receiver reconstructs the transmitted signals using well-known algorithms, such as maximum-likelihood estimation algorithms or minimum-mean-square algorithms. Since the reconstruction of STBC FDM signals are well known in the art, further discussion of signal reconstruction is omitted herein. In such STBC OFDM systems, digital data is encoded and transformed into analog RF signals suitable for transmission.
An example of a prior-art STBC OFDM system is shown in FIG. 1. As shown in FIG. 1, the system has a serial-to-parallel converter 110 that receives a frequency-domain serial data stream 105. The S/P converter 110 converts the frequency-domain serial data stream 105 into an N-dimensional frequency-domain parallel data block. The N-dimensional frequency-domain parallel data block is supplied to a frequency-domain STBC encoder 115 as X1 and X2 according to STBC convention. Upon receiving X1 and X2, the STBC encoder 115 generates two parallel frequency-domain digital signals XA 120 and XB 125 in a two-step process, such that XA=X1 and XB=X2 for the first process, and XA=−X2* and XB=X1* for the second process.
Upon generating the frequency-domain digital signals 120, 125 (or the appropriate complex conjugate signals), an IDFT is performed on the first frequency-domain digital signal XA 120 by an IDFT circuit 130. The IDFT of XA 120 produces a time-domain digital signal xA. The time-domain digital signal xA is converted to a time-domain serial data stream 150 by a parallel-to-serial converter 140. Thereafter, a cyclic prefix is added to the time-domain serial data 150 at a CP adder 160 to produce a cyclic-prefixed time-domain data stream xACP 170, which is relatively immune to ISI effects. This cyclic-prefixed time-domain data stream is converted to an analog RF signal for transmission by a radio frequency (RF) transmitter 180.
Similarly, the second digital signal XB 125 follows a similar path, and is cascaded through an IDFT circuit 135, a P/S converter 145, and a CP adder 165 to produce another cyclic-prefixed time-domain data stream xBcp 175. This cyclic-prefixed time-domain data stream is also converted to an analog signal for transmission by an RF transmitter 185. The two analog signals are transmitted substantially simultaneously from both RF transmitters 180, 185.
As shown in FIG. 1, since the frequency-domain STBC encoder 115 generates two frequency-domain digital signals XA 120 and XB 125 that are cascaded through two parallel paths, each hardware component in one path must have an analogous component in the other path. This duplication of hardware components results in added circuitry for each path, and, concomitantly, results in added computational complexity arising from the added circuitry.
The resulting complexity in computation and circuit give rise to a heretofore unaddressed need in the industry.